Currently, copper is being introduced in ULSI metallization schemes as a replacement for aluminum due to its lower resistivity and better electromigration resistance. Electroplated copper is becoming the method of choice for depositing copper layers for metallization schemes based on dual damascene back-end technology in ULSI processing.
One requirement of electroplating copper is to have a copper seed layer on top of the copper diffusion barrier layer in order to obtain uniform plating. Different techniques have been proposed in order to obtain an adequate copper seed layer.
Among these techniques is direct galvanic plating of copper from classical commercial copper electroplating baths. However, this technique has proved to be elusive, yielding non-uniform copper deposition on the barrier layer, with very bad adhesion and poor quality of the plated copper films (copper dust).
As an alternative, copper seed layers have traditionally been deposited by PVD techniques, such as IMP-Cu sputtering or long-throw sputtering. However, these techniques require to sputter copper seed thicknesses of around 150 nm or more in order to get sufficient copper coverage on the sidewalls of the features, due to the high aspect ratio of the features to be filled with copper (trenches and/or vias). Indeed, the sidewalls of the features have to be completely covered with copper, as the subsequent electroplating step critically depends on an uninterrupted path for current lines throughout the features. In the presence of sidewall defects due to interrupted copper seed layer deposition, large voids are observed after full copper layer plating. Also, the typical overhang of the 150-200 nm thick PVD copper layer at the entrance of narrow trench and/or via features can result in less than adequate filling with electroplated copper.
Therefore, there is a need for the future technologies to deposit thinner and more conformal copper seed layers on barrier layers in order to be able to fill the very narrow features with standard ECD copper.
In WO99/47731 a solution to this problem has been proposed, which is based on a PVD seed layer repair or mending technique using an electroplating process step after PVD seed layer deposition and prior to full copper plating. However, the plating mostly occurs on the already deposited PVD copper seed layer whereas the adhesion to the originally uncovered barrier layer (e.g. TaN) is expected to be very poor. This can possibly result in reliability problems after full metallization processing.
A possible option for more conformal seed layer deposition on barrier layers is Cu-CVD which results in substantially improved step coverage as compared to the PVD techniques. However, this technique has never gained wide acceptance in the field, most likely due to the high cost of the technology and the fairly rough copper films deposited with this technique.
Electroless copper is another possibility for deposition of copper layers. The principle of electroless metal deposition is based on the generation of electrons at a catalytically active or an activated surface in contact with a solution of metal ions in the presence of a suitable sacrificial electron donor. These electrons are capable of reducing the metal ions leading to the deposition of the metal on the activated surface. However, electroless plating baths have often a limited stability and can only be effectively used in a limited pH range which makes them very sensitive for slight variations in the composition of the plating solution. Such variations result in small variations in the pH but often lead to a large decrease in the deposition rate. Moreover, most electroless copper plating solution compositions are based on salts containing mainly sodium as the counterion. These high levels of sodium ions in the plating solutions can introduce severe reliability problems, particularly when sodium reaches the semiconductor device junctions, as this is known to be a production yield killer in semiconductor device manufacturing.
In conclusion the need for satisfying techniques to deposit thinner and more conformal copper seed layers on barrier layers still exists.